Clean, High-yield preparation of S,S and R,S amino acid isosteres

ABSTRACT

The present invention provides compounds and methods that can be used to convert the intermmediate halomethyl ketones (HMKs), e.g., chloromethyl ketones, to the corresponding S,S- and R,S-diastereomers. More particularly, the present invention provides: (1) reduction methods; (2) inversion methods; and (3) methods involving the epoxidation of alkenes. Using the various methods of the present invention, the R,S-epoxide and the intermediary compounds can be prepared reliably, in high yields and in high purity.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/132,278, filed May 3, 1999, which isincorporated herein by reference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

Human immunodeficiency virus (HIV), the causative agent of acquiredimmunodeficiency syndrome (AIDS), encodes three enzymes, including thewell-characterized proteinase belonging to the aspartic proteinasefamily, the HIV protease. Inhibition of this enzyme has been regarded asa promising approach for treating AIDS. Hydroxyethylamine isosteres havebeen extensively utilized in the synthesis of potent and selective HIVprotease inhibitors. However, this modern generation of HIV proteaseinhibitors has created an interesting challenge for the syntheticorganic chemist. Advanced x-ray structural analysis has allowed for thedesign of molecules that fit closely into active sites on enzymescreating very effective drug molecules. Unfortunately, these molecules,designed by molecular shape, are often difficult to produce usingconventional chemistry.

The modern generation of HIV inhibitors has structural similarities in acentral three-carbon piece containing two chiral carbons that link twolarger groups on each side (see, e.g., Parkes, et al., J. Org. Chem.1994 39, 3656). Numerous synthetic routes to these isosteres have beendeveloped. As illustrated below, a common strategy to prepare thelinking group starts with an amino acid, such as phenylalanine, to setthe chirality of the first carbon. Then, the linking group is completedby a series of reactions including a one-carbon homologization duringwhich the old amino acid carbon is transformed into ahydroxy-functionalized carbon having the correct chirality. However, thecommercial production of isosteres by this method presents seriouschallenges, generally requiring low-temperature organometallic reactions(Baragua, et al., J. Org. Chem. 1997, 62, 6080) or the use of exoticreagents.

A second approach, which is illustrated below, is to convert the aminoacid to an aldehyde and to add the carbon by use of a Wittig reaction togive an olefin (see, Luly, et al., J. Org. Chem. 1987, 52, 1487). Theolefin is then epoxidized. Alternatively, the aldehyde can be reactedwith nitromethane, cyanide (see, Shibata, et al., Chem. Pharm. Bull.1998, 46, 733) or carbene sources (see, Liu, et al., Org. Proc. Res.Dev. 1997, 1, 45). Instability and difficulty in preparation of thealdehyde make these routes undesirable (see, Beaulieu, et al., J. Org.Chem. 1997, 62, 3440).

Other routes that have been published, but not commercialized areillustrated in FIG. 1.

One of the best reagents that can be used to add a single carbon toamino acids is diazomethane because it gives high yields and fewside-products. In addition, diazomethane reactions are very clean,generating only nitrogen as a by-product. HIV inhibitor molecules needhigh purity because of the high daily doses required. As such,diazomethane is an ideal reagent for making high purity compounds. Inspite of the documented hazards of diazomethane, processes have recentlybeen developed that permit the commercial scale use of diazomethane toconvert amino acids to the homologous chloromethyl ketones (see, U.S.Pat. No. 5,817,778, which issued to Archibald, et al. on Oct. 6, 1998;and U.S. Pat. No. 5,854,405, which issued to Archibald, et al. on Dec.29, 1998). FIG. 2 illustrates examples of HIV protease inhibitorswherein the central linking group can be synthesized by the commercialuse of diazomethane. FIG. 3 illustrates a general reaction scheme thatcan be used to prepare the S,S-epoxide compound using diazomethane.

The most useful amino acid isosteres are based on phenylanaline. The keyintermediate in the synthesis of Sequinivir® (Roche) and Aprenavir®(Glaxo Wellcome) is the(S,S-)N-t-butoxycarbonyl-1,2-epoxy-4-phenyl-3-butanamine. Several otherprotease inhibitors, such as those described in Chen, et al. (J. Med.Chem. 1996, 39, 1991) or those under development (e.g., BMS-234475 orBMS-232623), use the diastereomeric (R,S-)N-t-butoxycarbonyl-1,2-epoxy-4-phenyl-3-butanamine.

Beginning with readily available (L)-phenylanaline, one is able tomanufacture N-t-butoxycarbonyl-1-chloro-2-keto-4-phenylbutanamine(called “chloroketone” or “CMK”) using the methods described in theliterature (see, e.g., Parkes, et al., J. Org. Chem. 1994 39, 3656;Shaw, E., Methods in Enzymology (Academic Press, New York, London),1967, 11, 677; and Dufour, et al., J. Chem. Soc. Perkin Trans. I 1986,1895, the teachings of which are incorporated herein by reference).However, what are needed in the art are methods that allow one toproduce reliably and in high-yields either diastereomer, i.e., the S,Sor the R,S, from the common chloroketone starting material (see, FIG.4). Quite surprisingly, the present invention fulfills this and otherneeds.

SUMMARY OF THE INVENTION

The present invention provides compounds and methods that can be used toconvert the intermmediate halomethyl ketones (HMKs), e.g., chloromethylketones, to the corresponding S,S- and R,S-diastereomers. It is thesechiral centers that determine the chiral centers in the HIV proteaseinhibitor and, thus, the efficacy of the drug. As explained herein, thepresent invention provides (1) reduction methods; (2) inversion methods;and (3) methods for preparing alkenes that, in turn, can undergoepoxidation reactions to form the desired R,S-epoxide. Using the variousmethods of the present invention, the R,S-epoxide and the intermediarycompounds can be prepared reliably, in high yields and in high purity.

As such, in one embodiment, the present invention provides a method forselectively preparing an R,S-halomethyl alcohol (R,S-HMA) compoundhaving the following general formula:

the method comprising: reducing a compound having the following generalformula:

with a non-chelating, bulky reducing agent to form the R,S-HMA compound.In the above formulae, R¹ is an amino acid side chain (e.g., a benzylgroup, an S-phenyl group, an alkyl group and a para-nitrobenzene group,etc.); R² is a blocking or protecting group (e.g., Boc, Cbz, Moc, etc.);and X¹ is a leaving group (e.g., a halo group, such as chloro). In apresently preferred embodiment, the non-chelating, bulky reducing agentis a member selected from the group consisting of LATBH and STBH. Inanother presently preferred embodiment, the reduction is carried out ina solvent such as diethyl ether. Once formed, the R,S-HMA can be reactedwith an alkali metal base to form an R,S-epoxide.

In another embodiment, the present invention provides a method forpreparing an R,S-halomethyl alcohol (R,S-HMA) compound having thefollowing general formula:

the method comprising: reducing a halomethyl ketone (HMK) compoundhaving the following general formula:

with a reducing agent selected from the group consisting of sodiumcyanoborohydride, cerium chloride/sodium borohydride, K-Selectride®,KS-Selectride® and (+)-Dip Chloride™ to form the R,S-HMA compound. Inthis method, R¹ is an amino acid side chain; R² is a blocking group; andX¹ is a leaving group. Again, once formed, the R,S-HMA can be reactedwith an alkali metal base to form an R,S-epoxide.

In another aspect, the present invention provides inversion methods thatcan be used to selectively prepare the R,S-epoxide. In one embodiment ofthe inversion method, R,S-epoxide is prepared by a four step process.More particularly, in one embodiment of the inversion method, thepresent invention provides a method for preparing an R,S-epoxide havingthe following general formula:

the method comprising: (a) reducing a halomethyl ketone (HMK) compoundhaving the following general formula:

with a reducing agent to form an S,S-halomethyl alcohol (S,S-HMA)compound having the following general formula:

(b) contacting the S,S-HMA compound of Formula II with a member selectedfrom the group consisting of arylsulfonyl halides and alkylsulfonylhalides in the presence of an amine to form an S,S-halomethyl sulfonyl(S,S-HMS) compound having the following general formula:

(c) contacting the S,S-HMS compound of Formula III with an acetate inthe presence of a phase transfer catalyst and water to form anR,S-halomethyl acetate (R,S-HMAc) compound having the following generalformula:

and (d) contacting the R,S-HMAc compound of Formula IV with an alkalimetal base to form the R,S-epoxide. In the above formulae, R¹ is anamino acid side chain (e.g., a benzyl group, an S-phenyl group, an alkylgroup, a para-nitrobenzene group, etc.); R² is a blocking or protectinggroup; X¹ is a leaving group (i.e., a halo group, such as chloro); R³ isa functional group including, but not limited to, arylsulfonyls andalkylsulfonyls (e.g., a mesyl group, a tosyl group, a triflate group, anosyl group, etc.); and R⁴ is an acyl group derived from the acetate(e.g., an acetyl group).

In another embodiment of the inversion method, the present inventionprovides a method for preparing an R,S-epoxide compound having thefollowing general formula:

the method comprising: (a) contacting an S,S-halomethyl sulfonyl(S,S-HMS) compound having the following general formula:

with a carbamate-forming acetate to form a cyclic carbamate; and (b)contacting the cyclic carbamate with an alkali metal base to form theR,S-epoxide. In the above formulae, R¹, R², R³ and X¹ are as definedabove. In a presently preferred embodiment, the carbamate-formingacetate is sodium trichloroacetate.

In yet another aspect, the present invention provides a method forpreparing R,S-epoxide by the epoxidation of an alkene. Moreparticularly, the present invention provides a method for preparing analkene having the following general formula:

the method comprising: (a) contacting a compound having the followinggeneral formula:

with a hydrohalo acid to form a compound having the following generalformula:

(b) reducing a compound of Formula II with a reducing agent to form acompound having the following general formula:

and (c) dehalohydroxylating a compound of Formula III to form thealkene. In the above formulae, R¹, R², and X¹ are as defined above. Onceprepared, the alkene can be converted to the R,S-epoxide using, forexample, m-chloroperbenzoic acid.

Other features, objects and advantages of the invention and itspreferred embodiments will become apparent from the detailed descriptionwhich follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates various routes that can be used to prepare anR,S-epoxide. FIG. 1(A) illustrates a method described by Ng, et al.,Org. Proc. Res. Dev. 1997, 1, 46; and Beaulieu, et al., J. Org. Chem.1997, 62, 3441. FIG. 1(B) illustrates a method described by Parkes, etal. J. Org. Chem. 1994, 59, 3656.

FIG. 2 illustrates examples of HIV protease inhibitors where the centrallinking group can be synthesized by commercial use of diazomethane.

FIG. 3 illustrates a general reaction scheme that can be used to preparethe epoxide compound.

FIG. 4 illustrates the two diastereomers that can be formed from thecommon chloroketone starting material, i.e., S,S-epoxide andR,S-epoxide.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The present invention provides various compounds and methods that can beused to prepare both reliable and in high yields either diastereomer,i.e., the S,S- or the R,S-, from the common halomethyl ketone (e.g.,chloromethyl ketone) starting material. More particularly, as explainedherein in greater detail, the present invention provides (1) reductionmethods; (2) inversion methods, and (3) methods involving theepoxidation of alkenes.

A. The Reduction Methods

A variety of reducing agents can be used to reduce a halomethyl ketone(HMK) to a halomethyl alcohol (HMA) (see, Table I). However, under mostconditions, the predominate diastereomer is the 2S,3S-HMA. For instance,reduction of HMK with sodium borohydride in ethanol (Chen, et al., J.Med. Chem. 1996, 39, 1991) produces a 1:4 mixture of R,S:S,S HMA in nearquantitative yield. Moreover, the reduction of HMK with aluminiumisopropoxide in isopropanol can give ratios as high as 1:18 in favor ofthe S,S-isomer (see, U.S. Pat. Nos. 5,684,176 and 5,847,144, both ofwhich issued to Hilpert). Thus, commercial routes to S,S-HMA are easilyachieved.

In contrast, the preparation of the R,S-isomer is much more difficult. Aslight increase in the R,S-HMA:S,S-HMA ratio is achieved when thereaction solvent, ethanol, is replaced with THF. Further enhancement inthe R,S-HMA:S,S-HMA ratio is obtained when the reduction is carried outin the presence of CeCl₃ (Barluenga, J. Org. Chem. 1997, 62,5974); buteven then the ratio of R,S-HMA:S,S-HMA is <1:1. Other reducing agents,such as LiAlH4, sodium cyanoborohydride, potassium borohydride, etc.,under a variety of reaction conditions, also fail to provide>1:1R,S-HMA:S,S-HMA. In fact, a perusal of the literature supports theobservation that S,S-HMA is the preferred isomer using coordinatingreducing reagents, such as borohydrides or aluminium hydrides (see, U.S.Pat. Nos. 5,684,176 and 5,847,144, both of which issued to Hilpert).

In contrast to the teachings of both the scientific and patentliterature, it has now been discovered that the reduction of HMKproceeds with high R,S diastereoselectivity when lithium aluminumt-butoxyhydride (LATBH) is used as the reducing agent. Quitesurprisingly and in contrast to the findings of the prior art, it hasbeen found that the reduction of HMK with LATBH in, for example,diethylether provides a 8:1 mixture of R,S-HMA:S,S-HMA in 97% yield.This high diastereofacial selectivity of the LATBH reducing agent isunusual since reduction of HMK with similar reducing agents, such aslithium aluminum hydride or sodium borohydride, do not favor R,Sdiastereoselectivity (see, U.S. Pat. Nos. 5,684,176 and 5,847,144, bothof which issued to Hilpert).

TABLE I HMK Reductions: Reagent(s) Solvent(s) Temp Time R,S:S,SLi(OtBu)3AIH Et2O  0 C.  3 Hrs   8:1 (+)-Dip Chloride ™(1.4eq) THF  5C.-RT 12 Hrs   5:1 K-Selectride ® THF Reflux  2 Hrs   2:1K-Selectride ®/Ti(OiPr)4 THF 25 C. 30 Min   2:1 KS-Selectride ® THF RT 2 Hrs   2:1 K-Select./MgBr2*OEt2 THF RT 30 Min 2.6:1 R-AlpineBorane(Conc.) THF Reflux  9 Dys   1:1 L-Selectride ® THF R.T.  1 Hr0.9:1 NaBH4/CeCl3(anh.) THF RT  2 Hrs 0.8:1 N-Selectride ® EtOH/THF R.T. 2 Hrs 0.7:1 NaBH4/CeCl3*7H2O THF 25 C. 18 Hrs 0.7:1NaBH4/EDTA(Na2*2H2O) THF RT 30 Min 0.7:1 NaCNBH3 THF RT 36 Hrs 0.7:1(+)-2-Butanol/NaBH4 THF RT  1 Hr 0.6:1 Cp2TiBH4 Glyme R.T. 30 Min 0.6:1NaBH4 THF 25 C.  2 Hrs 0.6:1 NaBH4/(−)-2-Butanol THF RT 30 Min 0.6:1NaBH4/Al(OiPr)4 THF Reflux  2 Hrs 0.6:1 NaBH4/DiacetoneDglucose THF RT12 Hrs 0.6:1 NaBH4/EDTA THF RT 12 Hrs 0.6:1 NaBH4/L-Tartaric Acid THF  5C.  1 Hr 0.6:1 NaBH4/MgBr2*OEt2 THF RT  1 Hr 0.6:1 BH3-t-butylamine THFR.T.  1 Hr 0.5:1 LAH THF 25 C.  1 Hr 0.5:1 LS-Selectride ® THF RT  1 Hr0.5:1 NaBH4/D-Tartaric Acid THF RT 30 Min 0.5:1 (+)-2-Butanol*BH3 THF RT 1 Hr 0.4:1 NaBH4/CaCl2 MeOH R.T.  1 Hr 0.4:1 AminoAlcohol Borane THF 25C. 12 Hrs 0.3:1 Na(PEG)2BH2 THF RT 30 Min 0.3:1 THF*BH3 EtOH/THF R.T.  2Hrs 0.2:1 Al(iOPr)3 IPA 50 C.  3 Dys 0.05:1  Na HB(OCH3)3 MeOH RT  1 Hr  1:1

As such, in one embodiment, the present invention provides a method forpreparing an R,S-halomethyl alcohol (R,S-HMA) compound having thefollowing general formula:

the method comprising: reducing a compound having the following generalformula:

with a non-chelating, bulky reducing agent to form the R,S-HMA compound.

In the above formulae, R¹ is an amino acid side chain. Moreparticularly, in the above formulae, R¹ is a side chain from any of thenaturally occurring amino acids or amino acid mimetics. In a preferredembodiment, R¹ is a benzyl group, a substituted benzyl group, anS-phenyl group, an alkyl group or a para-nitrobenzene group. In an evenmore preferred embodiment, R¹ is a benzyl group. R², in the aboveformulae, is a blocking or protecting group. It will be readily apparentto those of skill in the art that suitable -amino blocking groupsinclude, for example, those known to be useful in the art of stepwisesynthesis of peptides. Included are acyl type protecting groups (e.g.,formyl, trifluoroacetyl, acetyl, etc.), aromatic urethane typeprotecting groups (e.g., benzyloxycarboyl (Cbz), substituted Cbz, etc.),aliphatic urethane type protecting groups (e.g., t-butyloxycarbonyl(Boc), isopropylcarbonyl, cyclohexyloxycarbonyl, etc.) and alkyl typeprotecting groups (e.g., benzyl, triphenylmethyl, etc.). In a presentlypreferred embodiment, the blocking group is selected from the groupconsisting of Boc, Cbz and Moc (methoxycarbonyl). In the above formulae,X¹ is a leaving group. Suitable leaving groups will be readily apparentto those of skill in the art. In a presently preferred embodiment, theleaving group is a halo group (e.g., Cl, Br, F or I). In an even morepreferred embodiment, X¹ is a chloro or bromo group. Although many ofthe compounds disclosed herein contain the exemplar designation “halo,”such as halomethyl ketone (HMK) or halomethyl alcohol (HMA), it will bereadily apparent to those of skill in the art that other leaving groupscan be used in place of the halo group.

In the above embodiment, the reduction is carried out using anon-chelating, bulky reducing agent. It has surprisingly been discoveredthat non-chelating, bulky reducing agents favor the S,R-diastereomer.Examples of non-chelating, bulky reducing agents suitable for use in themethods of the present invention include, but are not limited to,lithium aluminum t-butoxyhydride (LATBH), sodiumtris-t-butoxyborohydride (STBH). In a presently preferred embodiment,the non-chelating, bulky reducing agent is LATBH. Once formed, theR,S-HMA can be reacted with an alkali metal base to form an R,S-epoxide.An exemplar embodiment of the above method is illustrated by thefollowing reaction scheme:

Synthesis of R,S-Boc-Epoxide by LATBH Reduction

In this embodiment, the reduction is preferably carried out in asolvent. It will be readily apparent to those of skill in the art thatnumerous solvents can be used. Exemplar solvents include, but are notlimited, to the following: diethyl ether, THF, MTBE and mixturesthereof. Quite surprisingly, it has been found that the reduction ofLATBH is dependent on the solvent employed. For instance, when diethylether is used as the solvent, a 8:1 mixture of R,S-HMA:S,S-HMA isobtained. However, when THF or MTBE is used as the solvent the ratio ofR,S-HMA:S,S-HMA is less than or equal to about 2:1. Based on theseresult, it is thought that a variety of factors, such as steric,solvation and chelation, are responsible for the high R,Sdiastereoselectivity observed in LATBH reduction of HMK. Thus, whenLATBH is used as the reducing agent, diethyl ether is preferably used asthe solvent.

LATBH is commercially available as a white powder and is used as asuspension in diethyl ether. Alternately, LATBH can be prepared in situby the reaction of LAH with 3 equivalents of t-butylalcohol indiethylether and then reacted with HMK. The best solvent, as judged onbasis of R,S-diastereoselectivity, is diethyl ether. However, thesolubility of HMK in diethyl ether is relatively low and a large amountof diethyl ether is needed to dissolve CMK, thereby reducing reactorefficiency to some extent. The reactor efficiency can be improved byeither adding HMK as a solid or, alternatively, as a solution in asecondary solvent (e.g., THF, toluene, ethyl acetate, etc.) to asuspension of LATBH in diethyl ether. The reaction rate is not affected,but the diastereoselectivity can be reduced from 8:1 in pure diethylether to about 5:1 with the above modifications.

In this embodiment, the reduction can be carried out at a temperatureranging from about −30° C. to about 25° C. In a presently preferredembodiment, the reduction is carried out at a temperature ranging fromabout −5° C. to about 5° C. At lower temperatures, larger amounts ofsolvent are needed to maintain homogeneity; whereas at hightemperatures, formation of the epoxide, resulting from intramolecularcyclization, is observed. At 0° C., the reduction reaction is rapid andis complete in less than about 30 minutes. It will be readily apparentto those of skill in the art that the progress of the reduction reactioncan be monitored by, for example, HPLC, and the reaction is deemedcomplete when the amount of unreacted HMK is less than about 1%.

In another embodiment, the present invention provides a method forpreparing an R,S-halomethyl alcohol (R,S-HMA) compound having thefollowing general formula:

the method comprising: reducing a halomethyl ketone (HMK) compoundhaving the following general formula:

with a reducing agent selected from the group consisting of sodiumcyanoborohydride, cerium chloride/sodium borohydride, K-Selectride®,i.e., potassium tri-sec-butylborohydride, KS-Selectride®, i.e.,potassium trisamylborohydride, and (+)-Dip Chloride™, i.e.,(+)-B-chlorodiisopinocampheylborane, to form the R,S-RMA compound. Inthis method, R¹ is an amino acid side chain; R² is a blocking group; andX¹ is a leaving group. It will be readily apparent to those of skill inthe art that the foregoing discussions relating to R¹, R² and X¹ andtheir preferred embodiments are fully applicable to this method and,thus, will not be repeated.

As with the previously described method, the reduction is preferablycarried out in a solvent. It will be readily apparent to those of skillin the art that numerous solvents can be used. Exemplar solventsinclude, but are not limited, to the following: diethyl ether, THF, MTBEand mixtures thereof. In a preferred embodiment, diethyl ether or THF isemployed as the solvent. Moreover, as with the previously describedmethod, the reduction can be carried out at a temperature ranging fromabout −30° C. to about 25° C. In a presently preferred embodiment, thereduction is carried out at a temperature ranging from about −5° C. toabout 5° C.

In yet another embodiment, the present invention provides a method forisolating an R,S-halomethyl alcohol (R,S-HMA) from a mixture of R,S-HMAand S,S-HMA. S,S-HMA is crystalline and is relatively easy to purify. Incontrast, the R,S-HMA is soluble in most organic solvents and isdifficult to purify by standard purification techniques, such asrecrystallization. Mixtures of R,S-HMA and S,S-HMA can be separated bycolumn chromatography or by preparative scale HPLC, but are notpractical economically.

It has now been discovered that a mixture of R,S-HMA and S,S-HMA can beseparated on the basis of differential solubility; R,S-HMA is soluble inhot hexanes, whereas the crystalline diastereomer, S,S-HMA, is not. Assuch, the present invention provides a method for isolating anR,S-halomethyl alcohol (R,S-HMA) from a mixture of R,S-HMA and S,S-HMA,the method comprising: combining the mixture of R,S- and S,S-HMAs withhexane and heating to a temperature ranging from 50° C. to about 60° C.to produce a hexane extractant; cooling the hexane extractant to atemperature ranging from about 0° C. to about 10° C., filtering thehexane extractant to form a first retentate and recovering the firstretentate; combining the first retentate with hexane to form a hexanesolution, heating the hexane solution to a temperature ranging fromabout 50° C. to about 60° C., and cooling the hexane solution to atemperature ranging from about 30° C. to about 40° C. to produce asuspension; and filtering the suspension to form a second retentate andrecovering the second retentate, wherein the R,S-HMA is present in thesecond retentate.

For instance, a crude reaction mixture, consisting of 50-90% R,S-HMA,10-50% S,S-HMA and 0-10% Me-ester, was extracted with hot hexane and theresulting hexane extractant was cooled to 10° C. and filtered to provideabout 94% pure R,S-HMA in 74% yield (based on HMK); the majorcontaminant was S,S-HMA (5%). Attempts to purify the 94% pure materialby differential solubility (above treatment) or by recrystallizationfrom a variety of solvent/solvent mixtures were not completelysuccessful. However, it has been determined that the best way to purifythe 94% pure R,S-HMA is to dissolve it in hot hexane (about 60° C.),cool to about 40° C., and then allowing the mixture to crystallize atabout 35° C. to about 37° C. for at least 2 h. The crystallized productis then filtered at about 30° C. to about 35° C. to provide about 99.5%pure R,S-HMA in 83% recovery. Interestingly, it has been found that ifthe mixture is cooled to 25° C. and filtered, a mixture consisting ofabout 94.5% R,S-HMA and 5.5% S,S-HMA, is obtained. This result issurprising because S,S-HMA is more crystalline and is not soluble inhexane, thus suggesting that S,S-HMA, not R,S-HMA, should be the firstto crystallize. Although a variety of solvent/solvent mixtures, such asmethanol, methanol/water, toluene, dibutyl ether, etc., have been usedto purify 94% pure R,S-HMA, the highest degree of purity/recovery isobtained with the hot hexane method of the present invention.

Once prepared and purified, the R,S-HMA can be converted into anR,S-epoxide. As such, in another embodiment, the present inventionprovides an A method for preparing an R,S-epoxide compound having thefollowing general formula:

the method comprising: reducing a haloketone (HMK) compound having thefollowing general formula:

with a non-coordinating reducing agent to form an R,S-haloalcohol(R,S-HMA) compound having the following general formula:

and contacting the R,S-HMA compound of Formula II with an alkali metalbase to form the R,S-epoxide compound. It will be readily apparent tothose of skill in the art that the foregoing discussions relating to R¹,R² and X¹ and their preferred embodiments are fully applicable to thismethod and, thus, will not be repeated. In a presently preferredembodiment, the non-coordination reducing agent is LATBH and thereduction is carried out in diethyl ether. In another presentlypreferred embodiment, the alkali metal base is selected from the groupconsisting of NaOH, KOH, LiOH, NaOCH₃, NaOCH₂CH₃ and KOtBu. In a furtherpreferred embodiment, KOH is the alkali metal base used. In anotherembodiment, calcium hydroxide can be used.B. The Inversion Method

In one embodiment of the inversion method, R,S-epoxide is prepared by afour step process illustrated below. More particularly, in oneembodiment of the inversion method, the present invention provides amethod for preparing an R,S-epoxide having the following, generalformula:

the method comprising: (a) reducing a haloketone (HMK) compound havingthe following general formula:

with a reducing agent to form an S,S-haloalcohol (S,S-HMA) compoundhaving the following general formula:

(b) contacting the S,S-HMA compound of Formula II with a member selectedfrom the group consisting of arylsulfonyl halides and alkylsulfonylhalides in the presence of an amine to form an S,S-halomethyl sulfonyl(S,S-HMS) compound having the following general formula:

(c) contacting the S,S-HMS compound of Formula III with an acetate inthe presence of a phase transfer catalyst and water to form anR,S-halomethyl acetate (R,S-HMAc) compound having the following generalformula:

and (d) contacting the R,S-HMAc compound of Formula IV with an alkalimetal base to form the R,S-epoxide. It will be readily apparent to thoseof skill in the art that the foregoing discussions relating to R¹, R²and X¹ and their preferred embodiments are fully applicable to thismethod and, thus, will not be repeated. In the above formulae, R³ is afunctional group including, but not limited to, arylsulfonyls andalkylsulfonyls. In a presently preferred embodiment, R³ is a memberselected from the group consisting of a methylsulfonyl group (i.e., amesyl group), a toluenesulfonyl group (i.e., a tosyl group), atrifluoromethanesulfonyl group (i.e., a triflate group) and apara-nitrobenzene sulfonyl group (i.e., a nosyl group). It will bereadily apparent to those of skill in the art that other leaving groupscan be used as R³ in place of the arylsulfonyl and alkylsulfonyl groups.R⁴, in the above formulae, is an acyl group derived from the acetate. Ina presently preferred embodiment, R⁴ is an acetyl group.

In the first step, i.e., step (a), a HMK is reduced with a reducingagent to form an S,S-HMA. In a preferred embodiment, the reducing agentis selected from the group consisting of sodium borohydride, lithiumaluminum hydride and sodium cyanoborohydride. In another preferredembodiment, step (a) is carried out in a solvent. Suitable solventsinclude, but are not limited to, ethanol, methanol, isopropanol, THF,diethyl ether, etc. The reduction can be carried out at a temperatureranging from about −30° C. to about room temperature and, morepreferably, at about −20° C. In a presently preferred embodiment, thereduction step is carried out using sodium borohydride in ethanol toprovide a 6:1 mixture of S,S-HMA:R,S-HMA in 98% yield. The S,S-isomer ishighly crystalline and can be easily purified by recrystallization toprovide>99.8% pure S,S-HMA in 80% yield.

In addition to the foregoing, HMA can also be prepared by MerwinPondroff Verley reduction of HMK. In this process, HMK is reacted withaluminum isopropoxide in refluxing IPA to give S,S-CMA in highdiastereoselectivity. Presumably, under these conditions, the reductionoccurs under chelation control and a mixture of S,S-HMA:R,S-HMA withratios as high as 20:1 is obtained (see, U.S. Pat. Nos. 5,684,176 and5,847,144, both of which issued to Hilpert).

In the second step, i.e., step (b), an S,S-HMA is reacted with anarylsulfonyl halide or an alkylsulfonyl halide in the presence of anamine to form an S,S-halomethyl sulfonyl (S,S-HMS). Suitable aminesinclude, but are not limited to, trialkylamines (e.g., trimethylamine,triethylamine, etc.), pyridine, 4-dimethylamino pyridine, etc. In apresently preferred embodiment, the amine is triethylamine. Step (b) canbe carried out in a variety of different solvents. Exemplar solventsinclude, but are not limited to, the following: chlorinated solvents(e.g., methylene chloride, dichloroethane, chlorotoluene, etc.),aromatic hydrocarbons (e.g., toluene, xylenes, etc.), ethyl acetate,ethers (e.g., THF, diethyl ether, etc.), etc. In another presentlypreferred embodiment, step (b) is carried out at a temperature rangingfrom about −30° C. to about 100° C. and, more preferably, from about 10°C. to about 70° C.

In a particularly preferred embodiment of step (b), the S,S-HMA isreacted with methanesulfonyl chloride in toluene in the presence of anequivalent amount of triethylamine to give the corresponding 2S,3S-CMAMesylate in 98% yield. The reaction is exothermic and is best conductedat a temperature ranging from about from about 10° C. to about 70° C.The crude mesylate is recrystallized from toluene to provide greaterthan 95% pure S,S-CMA Mesylate in near quantitative yield. However, inthe preferred process, S,S-CMA Mesylate is not isolated and the solutionof crude S,S-CMA mesylate in toluene is used, without purification, inthe next step, i.e., step (c). Although this mesylation step can beconducted in a variety of solvents, toluene is the preferred solventbecause it can be used in the next step, thereby eliminating a solventexchange step from the process.

In the third step, i.e., step (c), the S,S-HMS is reacted with anacetate in the presence of a phase transfer catalyst and water to from aHMAc. Suitable acetates for use in the present method include, but arenot limited to, the following: cesium acetate, potassium acetate,tetrabutylammonium acetate and sodium acetate. In a presently preferredembodiment, the acetate is cesium acetate. A variety of phase transfercatalysts (PTCs) can be used in carrying out step (c). Exemplar phasetransfer catalysts include, but are not limited to, crown ethers (e.g.,18-crown-6, dibenzo crown ether, etc.), quaternary ammonium salts andquaternary phosphonium salts (e.g., TATB, aliq. 336, etc.). In apresently preferred embodiment, the phase transfer catalyst is a crownether. The crown ether 18-crown-6 is particularly preferred because itallows for the production of R,S-HMAc with least amount of side product.Moreover, the rate of reaction with 18-crown-6 is much faster than withany of the other phase transfer catalysts. In addition, 18-crown-6 canbe easily removed from the product by a simple water wash.

Step (c), i.e., the displacement reaction, can be carried out in avariety of different solvents. Suitable solvents include, but are notlimited to, hydrocarbons (e.g., hexane, heptane, etc.), aromatichydrocarbons (e.g., toluene, xylene, benzene, etc.) and chlorinatedsolvents (e.g., CCl₄, dichloroethane, chlorotoluenes, etc.). In apresently preferred embodiment, toluene is used as the solvent becauseit can be used for both steps (b) and (c), and it can be used as acrystallization solvent for the R,S-HMAc. In addition, toluene iscommercially available from a variety of sources and can be recycled inhigh efficiency. The displacement reaction, i.e., step (c) can becarried out at a temperature ranging from about 20° C. to about 100° C.In a presently preferred embodiment, the displacement reaction iscarried out at a temperature ranging from about 20° C. to about 100° C.

In addition to the foregoing, it has been found that the displacementreaction is dependent on the amount of water present in the reactionmixture. Presumably, a small amount of water is needed to overcome thelattice energy of the metal acetate, thereby making the nucleophileaccessible for the displacement reaction. However, it has been foundthat increased amounts of water will reduce the reactivity of thenucleophile by solvating it. Thus, in a preferred embodiment, the wateris maintained between about 0.5% and about 10.0% and, more preferably,between about 0.5% and about 5%. Once the displacement reaction iscompleted, the crude product can be isolated by crystallization from,for example, toluene/heptane to give typically greater than 99.5% pureR,S-HMAc in high yield. Alternately, the R,S-HMAc can be isolated andthen recrystallized from, for example, methanol/water to give pureR,S-HMAc.

In the final step of the above method, i.e., step (d), the R,S-HMAc isreacted with an alkali metal base to form the R,S-epoxide. It has beenfound that hydrolysis of the R,S-HMAc followed by subsequentintramolecular ring closure provides the R,S-epoxide in nearquantitative yield. In a presently preferred embodiment, the alkalimetal base is selected from the group consisting of NaOH, KOH, LiOH,NaOCH₃, NaOCH₂CH₃ and KOtBu. In another preferred embodiment, step (d)is carried out is a solvent. Suitable solvents include, but are notlimited to, hydrocarbons, aromatic hydrocarbons, chlorinated solventsand ethers (e.g., THF). In a presently preferred embodiment, the solventis a mixture of toluene and THF.

In a particularly preferred embodiment of step (d), the R,S-HMAc isreacted with aqueous potassium hydroxide (KOH) in a mixture of THF andethanol. Evaporation of solvent followed by tituration of the crudeproduct with hexane afforded the desired R,S-epoxide as a low melting,white solid.

Since the R,S-epoxide is soluble in most solvents, it is difficult topurify. In addition, the R,S-epoxide is reactive towards ring openingreactions and will react with potassium hydroxide in ethanol to give thecorresponding glycol or the ethoxyglcyol side products. Using thismethod of the present invention, high purity R,S-epoxide (>>99.5%) hasbeen prepared by incorporating the purity at the R,S-HMAc stage and thenmaintaining the purity by minimizing side reactions in the final step.Thus, it is important that the above conversion is achieved in nearquantitative yield and without formation of side products. Again, in apreferred embodiment of this method, this is accomplished by employingaqueous KOH. Presumably, in this form, the hydroxide is nucleophilicenough to allow hydrolysis to occur, but is not nucleophilic enough toreact with the R,S-epoxide and form side products.

Using this method of the present invention, greater than 99.5% pureR,S-epoxide can be prepared in 95-97% yields. The R,S-epoxide preparedby this process can be characterized by NMR, HPLC, TLC and DSC.Moreover, despite difficulties encountered in the prior art relating tothe purification of the R,S-epoxide, it has now been discovered that theR,S-epoxide can be purified by recrystallization from petroleum ether.This is an important discovery because traditional purificationtechniques, such as chromatography, are not applicable due toinstability of the R,S-epoxide towards silica gel and alumina. As such,in a preferred embodiment, the above method further comprises: purifyingthe R,S-epoxide by recrystallization with petroleum ether. An exemplarembodiment of the above method is illustrated by the following reactionscheme:

Preparation of the R,S-Epoxide Using One Embodiment of the InversionMethod

In another embodiment of the inversion method, the present inventionprovides a method for preparing an R,S-epoxide compound having thefollowing general formula:

the method comprising: (a) contacting an S,S-halomethyl sulfonyl(S,S-HMS) compound having the following general formula:

with a carbamate forming acetate to form a cyclic carbamate having thefollowing general formula:

and (b) contacting the cyclic carbamate with an alkali metal base toform the R,S-epoxide. It will be readily apparent to those of skill inthe art that the foregoing discussions relating to R₁ R², R³ and X¹ andtheir preferred embodiments are fully applicable to this method and,thus, will not be repeated. A “carbamate-forming acetate,” as usedherein, refers to an acetate that contains a sufficient leaving group.Exemplar carbamate-forming acetates include, but are not limited to,sodium trichloroacetate, potassium trichloroacetate, tetrabutylammoniumtrichloroacetate, sodium tribromoacetate, potassium tribromoacetatesodium trifluoroacetate and potassium trifluoroacetate.

As with the previously described methods, step (a) can be carried out ina variety of solvents, such as hydrocarbons (e.g., hexane, heptane,etc.), aromatic hydrocarbons (e.g., toluene, xylene, benzene, etc.) andchlorinated solvents (e.g., CCl₄, dichloroethane, chlorotoluenes, etc.).In a preferred embodiment, the solvent is toluene. In step (b) of theabove method, the cyclic carbamate is reacted with an alkali metal baseto form the R,S-epoxide. In a presently preferred embodiment, the alkalimetal base is selected from the group consisting of NaOH, KOH, LiOH,NaOCH₃, NaOCH₂CH₃ and KOtBu. In another preferred embodiment, step (b)is carried out in a solvent. Suitable solvents include, but are notlimited to, hydrocarbons, aromatic hydrocarbons, chlorinated solventsand ethers (e.g., THF). In a presently preferred embodiment, the solventis a mixture of THF and ethanol.

In connection with the above method, the present invention provides acyclic carbamate compound having the following general formula:

In the above formula, R¹ is an amino acid side chain (e.g., benzyl); R²is hydrogen or a blocking/protecting group (e.g., BOC, MOC, CBZ, etc.);and X¹ is a leaving group (e.g., a chloro or bromo group). This compoundcan be readily synthesized and purified using the methods set forth inExample II.

C. The Alkene Method

In another embodiment, the present invention provides a method forpreparing an alkene having the following general formula:

the method comprising: (a) contacting a compound having the followinggeneral formula:

with a hydrohalo acid to form a compound having the following generalformula:

(b) reducing a compound of Formula II with a reducing agent to form acompound having the following general formula:

and (c) dehalohydroxylating a compound of Formula III to form thealkene. It will be readily apparent to those of skill in the art thatthe foregoing discussions relating to R¹, R², and X¹ and their preferredembodiments are fully applicable to this method and, thus, will not berepeated.

In step (a), a compound of Formula I is reacted with a hydrohalo acid toform a compound of Formula II. Suitable hydrohalo acids include, but arenot limited to, hydrobromic acid, hydrochloric acid and hydroiodic acid.In a presently preferred embodiment, the hydrohalo acid is hydrobromicacid or hydrochloric acid. Step (b) can be carried out using any of avariety of reducing agents. In a presently preferred embodiment, sodiumborohydride is the reducing agent employed in step (b). Finally, in step(c), compound III is dehalohydroxylated to form the desired alkene.Suitable dehalohydroxylating compounds include, but are not limited to,zinc (0) metals (e.g., zinc dust), nickel metals, zinc mercury amalgan,etc. Step (c) can be carried out in a number of different solvents.Suitable solvents include, but are not limited to, methanol, ethanol,isopropanol, THF, MTBE, toluene, etc. In a presently preferredembodiment, zinc dust in ethanol is used in step (c).

Once prepared, the alkene can be converted to the R,S-epoxide using, forexample, m-chloroperbenzoic acid as illustrated below.

In one particularly preferred embodiment of this method, reaction of thediazoketone (i.e., the compound of Formula I), which is prepared fromphenylalanine using diazomethane, with hydrobromic acid gives thebromoketone (i.e., the compound of Formula II) in 77% yield. Reductionof the bromoketone with sodium borohydride under conditions similar tothose used for the chloroketone gave high selectivity for theS,S-bromomethylalcohol (i.e., the compound of Formula III) over theR,S-diastereomer. The desired S,S-isomer was isolated in 85% yield afterrecrystallization (see, Parkes, et al., J. Org. Chem. 1994 39, 3656).

The bromomethylalcohol was dehalohydroxylated to give the olefin (i.e.,the compound of Formula V) by zinc metal in ethanol. Upon work up, thet-BOC protected S-3-amino-4-phenyl-1-butene was isolated in 77% yield.Using this method of the present invention, very pure material wasprepared without the problems of racemization associated with thereaction of the t-BOC protected S-phenylalanal route. the alkene wasconverted to the R,S-epoxide using, for example, a published route usingm-chloroperbenzoic acid. An exemplar embodiment of the above method isillustrated by the following reaction scheme:

Preparation of the R,S-Expoxide Using the Alkene Method

The invention will be described in greater detail by way of specificexamples. The following examples are offered for illustrative purposes,and are not intended to limit the invention in any manner. Those ofskill in the art will readily recognize a variety of noncriticalparameters that can be changed or modified to yield essentially the sameresults.

EXAMPLES A. Example I

This example illustrates the preparation of S,S-CMA and R,S-CMA usingthe reduction methods of the present invention.

1. Preparation of S,S-CMA by Reduction

A 500 mL, 3-necked round bottom flask was fitted with a condenser,thermocouple temperature probe, dry nitrogen inlet, and magneticstirring. A stirred solution of chloromethylketone (CMK) (19.22 g,0.0645 mol) and Isopropanol (200 mL) was heated to 50° C. and aluminumisopropoxide (6.87 g, 0.0337 mol, 1.5 eq) was charged to the reactor.The reaction mixture was heated at 50° C. for three hours at which pointHPLC analysis indicated 0.4% CMK remained. After heating for 1additional hour and cooling to room temperature, the reaction wasquenched with water (200 mL) and glacial acetic acid (˜50 mL) to adjustthe pH to 4. The reaction was transferred to a separatory funnel and theorganic solids were extracted into ethyl acetate, resulting in two clearphases. The phases were split and the organic phase was evaporated to18.63 g (97% yield) off-white solid. S,S-Chloromethylalcohol (S,S-CMA):¹H NMR (CDCl₃): δ 1.37 (s, 9H), 2.97 (m, 2H, J=5.1 Hz), 3.20 (br d, 1H),3.55-3.69 (m, 2H), 3.83-3.93 (m, 2H), 4.59 (br d, 1H, J=6.6 Hz),7.21-7.34 (m, 5H); HPLC (Short) t_(R) 3.84 min=99.51%, 4.66 min=0.49%;HPLC (long) t_(R) 13.26 min=99.50%, 17.42 min=0.50%.

Proton NMR analysis of final product indicated ˜37:1 ratio of S,S:R,SBoc-phenylalanine Chloromethylalcohol (CMA), and traces of acetic acid.HPLC analysis indicated ˜32:1 ratio of S,S:R,S CMA (95.1% S,S CMA, 3.0%R,S CMA, 0.6% CMK, and 1.3% impurities from the starting material e.g.methyl ester, boc-phenylalanine). Further purification was accomplishedby recrystallization from heptane.

2. Sodium Cyanoborohydride Reduction of CMK

To a solution of sodium cyanaoborohydride (5.28 g, 84.0 mmol, 1.0 eq) inTHF (25 mL) was added a solution of CMK (25.0 g, 84.0 mmol) in THF (100mL), followed by addition of AcOH (10 mL) over 0.5 h at RT. During thisaddition, internal temperature was never allowed to rise above 42° C.After 1.5 h, TLC analysis of an aliquot indicated total consumption ofCMK signaling reaction completion. The reaction mixture was quenchedwith H₂O (250 mL) and the resulting white slurry was stirred at ambienttemperature for 1 h. The mixture was extracted with ethyl acetate (500mL) and then concentrated on a rotary evaporator to a volume of ca. 300mL. Water (100 mL) and the remaining ethyl acetate was removed underreduced pressure at 45° C. The precipitated product was filtered, washedwith water (200 mL), and dried in a vacuum oven at 45° C./28 inch-Hg for15 h to give 23.8 g (95% yield) of a white solid. HPLC analysis revealedthat the solid contained a mixture of 41% R,S-CMA and 59% S,S-CMA.

3. Preparation of R,S-CMA by Reduction With Cerium Chloride/SodiumBorohydride.

A 5000 mL, 3-necked round bottom flask was fitted with mechanicalstirring, Claisen head adapter, condenser, dry nitrogen inlet, glassenclosed thermocouple temperature probe, and solids addition funnel, alloven dried at 120° C. and cooled under dry nitrogen. To a stirred slurryof CMK (200 g, 0.672 mol, 1.0 eq), cerium chloride heptahydrate (250 g,0.672 mol, 1.0 eq), and THF (716 g) was added sodium borohydride (25.5g, 0.673 mol, 1.0 eq) portionwise over 70 minutes during which time a4.5° C. exotherm was observed. The reaction mixture was stirred for anadditional 5 hours at room temperature, at which time HPLC analysisindicated that starting material had been consumed. The reaction wascooled to 2° C. and ethyl acetate (500 mL) was added. The reaction wasquenched with water (1000 mL) at a rate to control the production ofhydrogen gas and maintain at a temperature of less than 20° C. The pH ofthe reaction was adjusted to approximately 6 with glacial acetic acid(18 mL) and additional ethyl acetate (2500 mL) was added to dissolve thesolids. The reaction was warmed to room temperature and transferred to a6000 mL separatory funnel. The organic phase was separated, washed withwater and evaporated in vacuo to give 175 g (96% yield) of a whitesolid. HPLC analysis of the solid indicated 36% R,S boc-phenylalaninechloromethylalcohol (CMA) and 60% S,S CMA; ¹H NMR analysis confirmed a0.6:1 R,S:S,S CMA ratio.

4. Preparation of R,S-CMA by Reduction with LATBH.

Lithium tri-t-butoxyaluminohydride (LATBH) (93.87 g, 0.369 mol, 1.1 eq)and anhydrous diethyl ether (500 mL) were placed in a reactor and cooledto 2° C. A solution of CMK (99.84 g, 0.355 mol) and anhydrous diethylether (2000 mL) was added over 90 min maintaining an internaltemperature of less than 5° C. After the addition was complete, themixture was stirred for 30 min at which point HPLC analysis indicated nostarting material remaining. The reaction was slowly quenched water(1500 mL) and then acetified with glacial acetic acid (1000 mL) at arate such the temperature was below 10° C. The reaction was warmed toambient and the organic phase was separated, washed with water and wasevaporated in vacuo to give an orange oil (100.12 g). Hexanes (500 mL)was added to the flask and evaporated on the rotary evaporator to removeresidual t-butanol and isobutanol; the evaporation yielded an orangeoil/solid (97.34 g, 97% yield).

HPLC and ¹H NMR analysis indicated an approximately 6.5:1 ratio ofR,S:S,S CMA. The R,S-isomers was purified by extraction into refluxinghexanes (300 mL), filtration while hot to remove the less solubleS,S-isomer, and slow cooling overnight. After filtration and drying,74.5 g (82.3% yield) of a product that was 92.1% R,S CMA and 5.4% S,SCMA by HPLC and ¹H NMR analysis.

5. Purification of Mixtures of S,S- and R,S-CMA

CMA (170 g of a mixure of 0.6 to 1 isomers) and hexanes (800 g) werecharged to the flask and heated to reflux for 1 hour. The less solubleisomer mix (90% S,S CMA, 9% R,S CMA) (99.6 g, 58% yield) was removed byfiltration of the hot mixture. The filtrate was evaporated to 75%volume, cooled and filtered to give the more soluble isomer mix (94% R,SCMA, 3% S,S CMA) 36.7 g (22% yield) were removed by cold filtrationthrough a 600 mL coarse, sintered glass funnel. The residual filtratewas dryed in vacuo to give a yellow oil (18.6 g, 11% yield) containing amixture of isomers.

A mixture of 32 g of the crude solid (93% R,S-CMA and 6% S,S-CMA) fromthe hot hexane recrystallization and hexanes (600 mL) was heated to 60°C. The resulting solution was slowly allowed to cool to 53° C. andseeded with R,S-CMA crystals. Further crystallization was observed at37° C. at which point significant amount of white needles had formed insolution. The internal temperature was maintained between 35-40° C. for1.5 h, at which point the mixture was hot filtered to provide 25.7 g(80% recovery) of R,S-CMA as white needles. HPLC analyses revealed thatR,S-CMA was 99.8% pure and contained ca. 0.2% S,S-CMA. Concentration ofhexane filtrate on a rotary evaporator afforded 6.1 g of a white solidwhich based on HPLC analysis was found to be consist of 91.9% R,S-CMAand 6.4% S,S-CMA.

R,S-Chloromethylalcohol (R,S-CAM): ¹H NMR (CDCl₃): δ 1.36 (s, 9H), 2.94(m, 2H, J=7.3 Hz), 3.54 (d, 2H, J=4.6 Hz), 3.77 (m, 1H, J=2.1 Hz), 3.94(1H, J=7.3 Hz), 4.99 (d, 1H, J=8.8 Hz), 7.24 (br m, 5H); HPLC (Short)t_(R) 3.87 min=0.21%, 4.69 min=99.79%.

B. Example II

This example illustrates the preparation of R,S,-Epoxide using twodifferent inversion methods. In NMR: Varian 300 MHz; HPLC: HewlettPackard 1100, column C18 reverse phase using acetonitrile/water withphosphate buffer; melting points were measured by DSC

1. Preparation of R,S-Epoxide by the Inversion Route Via an Acetate

a. Step 1: Mesylation

A 3 L jacketed reactor equipped with a mechanical stirrer, additionfunnel, reflux condenser, temperature probe, and a nitrogen gas inletwas charged with S,S-CMA (150.3 g, 0.501 mol) and toluene (1.5 L). Thesystem was flushed with nitrogen and triethylamine (62 g, 0.613 mol) wasadded. The resulting mixture was treated, dropwise, with methanesulfonylchloride (69 g, 0.595 mol). The rate of addition of methanesulfonylchloride was maintained so as to control the reaction temperature below50° C. When the addition was complete, the reaction mixture was stirredfor 1 h, sampled and analyzed by HPLC which indicated that the reactionwas complete. The reaction mixture was slowly quenched into 10% aqueouspotassium bicarbonate solution, and the organic phase was separated andwashed with water. The organic layer containing the mesylate derivativewas then dried azeotropically and used without isolation in thedisplacement reaction. In order to obtain yield/purity data, a sample ofreaction mixture was withdrawn and stripped off solvent under reducedpressure to give S,S-CMA mesylate, a pale yellow solid: mp 117-121° C.;¹H NMR (CDCl₃): δ 1.35 (s, 9H), 2.79 (br t, 1H, J=11.1 Hz), 3.04 (dd,1H, J=14.4, 4.8 Hz), 3.17 (s, 3H), 3.73 (m, 2H, J=4.5 Hz), 4.15 (ddd,1H, J=5.1, 4.8, 3.6 Hz), 4.69 (br d, 1H, J=6.6 Hz), 5.04 (br s, 1H),7.20-7.34 (m, 5H); HPLC revealed that the product was 99.7% (area %)pure.

b. Step 2: Displacement

A second reactor was charged with cesium acetate (241.7 g, 1.125 mol)and 18-crown-6 (33 g, 0.125 mol) in toluene (400 mL) and the mixture washeated to 70 C. Next, a solution of S,S-CMA mesylate in toluene wasadded over 1 h and the resulting mixture was heated at 70° C. for anadditional 9 hrs at which time TLC analysis indicated the reaction wascomplete. The reactor was cooled to 35° C., and water (1 L) was added.The organic layer was separated and washed with water and the solventwas evaporated until the concentration of the product was 20% by weightas determined by 1 H NMR analysis. Heptane (1350 g) was added and themixture heated to 55° C. for 30 min, and cooled to ambient over 1 h. Themixture was then cooled to 5° C., filtered, and the white solid wasdried in vacuo to give 131.5 g (77% yield) of(2R,3S)-N-t-butoxycarbonyl-1-chloro-2-acetoxy-4-phenylbutanamine, awhite solid: mp 105-106° C.; ¹H NMR (CDCl₃): δ 1.39 (s, 9 H), 2.13 (s,3H), 2.75 (br d, 2H, J=7.5Hz), 3.56 (br d, 2H, J=6.3Hz), 4.24 (ddd, 2 H,J=7.4, 2.2Hz), 4.52 and 4.67 (both br d, 1H total, J=9.6Hz), 5.03-5.12(m, 1H, J=2.1Hz), 7.17-7.33 (m, 5H); TLC (silica gel, 30% EtOAc/Hexane):R_(f)=0.75; HPLC analysis revealed that the product was 99.7% pure.

c. Step 3: Hydrolysis and Ring Closure

A 1 L flask fitted with a mechanical stirrer, addition funnel,temperature probe, and a nitrogen inlet was charged with R,S-CMA Acetate(34.3 g, 100.4 mmol), THF (156 mL), ethanol (90 mL) and water (30 mL).The mixture was cooled to 0-3° C. and a 43% aq. KOH solution (13.3 g of86% potassium hydroxide dissolved in 13.3 mL of water) was addeddropwise to the reaction mixture so as to maintain an internaltemperature of <5° C. The reaction mixture was stirred at 0-3° C. for1.5 h and then quenched with 6% aq. sodium biphosphate solution (250mL); the reaction temperature was maintained below 10° C. during quench.Diethyl ether (260 mL) was added and the organic layer was separated,dried (Na₂SO₄), filtered, and stripped of solvent under reduced pressureto give a clear oil. Hexane (130 mL) was added and the resulting mixturewas concentrated on a rotary evaporator till <10% hexane remained andthe residue was seeded with crystals of pure R,S-Epoxide. The mixturewas then stored at room temperature for 16 h and the precipitated solidwas collected by filtration and dried to provide 25.4 g (96%) of thetitle compound, a white solid: mp (DSC): 51.56° C.; ¹H NMR (CDCl₃): δ1.39 (s, 9H), 2.59 (s, 1H), 2.70 (dd, 1H, J=3.9 Hz), 2.91 (m, 2H,J=6.6Hz), 3.01 (m, 1H, J=3.6Hz), 4.13 (d, 1H, J=7.8 Hz), 4.49 (d, 1 H,J=7.2Hz), 7.27 (br m, 5H). The purity, as determined by HPLC analysis,was 99.5%.

d. Alternate Process for Preparation of 2R,3S-Chloromethylacetate

A 4 L jacketed reactor equipped with a mechanical stirrer, refluxcondenser, temperature probe, and a nitrogen gas inlet was charged withS,S-CMA Mesylate (246.5 g, 0.65 mol) and 18-crown-6 (43.4 g, 0.16 mol),cesium acetate (322.8 g, 1.685.7 mol) and toluene (3.2 L). The resultingmixture was heated at 72° C. for 11 hours, at which point TLC analysis(silica gel, 30% EtOAc/Hexane) indicated the starting material had beenconsumed. The organic phase was separated and concentrated under reducedpressure to provide a white solid. The residue was dissolved in ethylacetate (1.2 L) and the resulting solution was washed with H₂O (2×550mL), dried (Na₂SO₄), filtered, and stripped off solvent under reducedpressure to provide 216 g (97%) of 92% pure R,S-CMA Acetate.Recrystallization of the crude product from 85:15 methanol/waterprovided 99.7% pure R,S-CMA Acetate in 57% yield. The mother liquor wasconcentrated on rotary evaporator, treated with water, and chilled to 5°C. to provide an additional 22 g of 98.2% pure product, thus increasingthe total yield of R,S-CMA Acetate to 76%.

2. Preparation of R,S-Epoxide by the Inversion Route Via TrichloroaceticAcid

a. Step 1: Preparation of ‘Cyclic Carbamate’

A 250 mL round-bottom flask equipped with a magnetic stir bar, refluxcondenser, temperature probe, and a nitrogen gas inlet was charged with9.98 g (26.4 mmol) of S,S-CMMs, 0.434 g (1.35 mmol) oftetrabutylammonium bromide (TBAB), 7.46 g (40.2 mmol) of sodiumtrichloroacetate, and flushed vigorously with N₂. Toluene (104 mL, 90 g)was added under a steady stream of N₂ and the resulting slurry washeated to ˜45° C. The reaction mixture was stirred at 45° C. overnight,at which point TLC analysis (silica gel, 30% EtOAc/Hexane) indicated thestarting material had been consumed. The toluene phase was transferredfrom the reaction vessel into a 500 mL separatory funnel and EtOAc/H2O(50 mL/100 mL), used to rinse the reactor, was combined with the organiclayer. After separating the two layers, the organic layer was washedwith H₂O (1×100 mL), dried over Na₂SO₄, filtered, and removed undervacuum. The resulting crude solid was dried in a vacuum oven (45° C.)overnight to provide a yield of 92% (7.92 g, 24.3 mmol, ˜90% pure).

This product was combined with the crude cyclic carbamate (1.67 g, 5.13mmol) from a previous small scale synthesis (CP078-24) and crystallizedfrom MeOH/H₂O as follows: 9.59 g of crude product was dissolved in 43 mL(34 g) of MeOH while heating to 45° C. To this warm MeOH solution wasslowly added 4 mL of H₂O and the temperature allowed to reach ambientwithout agitation. Needle formation was rapid and the flask was cooledto 0-5° C. prior to filtration, yielding 7.24 g (75.5% recovery) ofproduct (99.42% pure).

b. Step 2: Preparation of R,S-Epoxide

To a 50 mL round-bottom flask equipped with a magnetic stir bar,temperature probe, and a nitrogen inlet was added a 43% aqueous KOHsolution (0.73 g soln., 5.82 mmol) and 1.0 g of H₂O. The contents of theflask were cooled to 0-3° C. with the aid of an ice-bath. A separateflask was charged with 0.99 g (2.22 mmol) of the ‘cyclic carbamate’, 3.2g of THF, and 1.6 g of EtOH and agitated to dissolve all solids. The‘cyclic carbamate’ solution was added dropwise to the reaction flask viapipet so as to maintain an internal temperature of <4° C. Once additionwas complete, the reaction was stirred at 0-3° C. for ˜1 hour, at whichpoint the reaction was quenched by addition of a sodium biphosphatesolution (0.448 g NaH₂PO₄, 6.8 g H₂O). The reaction quench was conductedat such a rate as to keep the internal temperature <10° C. (Note: Thereaction was analyzed for completion via TLC after a 30 min. post-stirand found to contain the desired epoxide.) The cloudy reaction mixturewas diluted with 10 mL of Et₂O and the layers were separated. The clearorganic layer was dried over Na₂SO₄, filtered, and the solvent wasremoved under vacuum to afford a clear oil (0.8 g).

The crude product was taken up in 20% EtOAc/hexanes (due to solubilityproblems in desired eluent) and purified via column chromatography(silica gel, 10% EtOAc/hexanes). R,S-epoxide, as well as a small amountof a nonpolar impurity, were collected prior to running a gradient to50% EtOAc/hexanes to collect the deblocked impurity. The two fractionswere evaporated of solvent to obtain clear oils: R,S-epoxide: 0.444 g(solidified under vacuum; HPLC: ˜90%). The identity of the R,S-epoxidewas confirmed by ¹H NMR, HPLC, and TLC.

c. Mechanistic Discussion

Without intending to be bound by any theory, it is thought that thereaction occurs through the following mechanism. Attack of atrichloroacetate anion on the secondary mesylate in an SN₂ fashioninverts the stereochemistry and provides the intermediateR,S-chloromethyltrichloroacetate (R,S-CMAcCl₃). Due to the excellentleaving group ability of: CCl₃ (trichlorocarbene), nucleophilic attackof the carbamate nitrogen on the acetate carbonyl and subsequent (orconcurrent) loss of a proton provides the cyclic carbamate. It isthought that treatment of this species with aqueous base favors reactionof the hydroxide at the cyclic carbonyl, possibly due to the addedbenefit of relieving the ring strain of the molecule, resulting in theexpected epoxide (55-60%).

C. Example III

This example illustrates the preparation of the R,S-epoxide by theepoxidation of an alkene.

1. Preparation of Bromomethyl Ketone (BMK):

A solution of diazomethyl ketone (DMK) in ethyl acetate/diethyl ether(16.8 g solution, 1 g DMK, 3.5 mmol) was cooled to 5° C. and treateddropwise with a solution of hydrobromic acid (1.8 g, 10.6 mmol); thereaction temperature was maintained below 10° C. during the addition.The resulting mixture was stirred at 0-5° C. for 2 hours and quenchedwith water (20 mL). The organic layer was separated and washed withwater (3×20 mL) until the pH of the final water wash was >6. The organiclayer was concentrated on a rotary evaporator to give 0.92 g (77%) of anoff-white solid. The product purity, as determined by HPLC, was 91%. ¹HNMR-(S,S-BMK; CDCl₃)): δ 1.41 (s, 9H), 3.07 (m, 2 H, J=6.6Hz), 3.94 (m,2H, J=16.2Hz), 4.72 (q, 1H, J=7.2Hz), 5.07 (d, 1H, J=7.5 Hz), 7.20-7.31(br m, 5H).

2. Preparation of Bromomethylalcohol (BMA):

A mixture of bromomethylketone (20.3 g, 59.3 mmol), ethyl acetate (160mL), and ethanol (240 mL) was cooled to −30° C. and treated, dropwise,with a slurry of sodium borohydride (1.16 g, 30.7 mmol) in ethanol (80mL). The reaction mixture was stirred at −30° C. for 30 min. andquenched with acetic acid (4 mL); the reaction temperature wasmaintained below −20° C. during the quench. The reaction mixture wasthen warmed to room temperature and treated with water (100 mL) andethyl acetate (150 mL). The layers were separated and the organic layerwas filtered to give 2.8 g of 96.6% pure S,S-BMA. The organic layer wasthen dried (Na₂SO₄), filtered, and evaporated in vacuo to give 14.2 g ofa mixture consisting of 85% S,S-BMA, 6% R,S-BMA, and 5% methyl ester. ¹HNMR (S,S-BMA; CDCl₃): δ 1.36 (s, 9H), 2.98 (br m, 2H, J=4.5Hz), 3.46 (brm, 1H, J=9Hz), 3.54 (br m, 1H), 3.86 (br s, 2H), 4.56 (br s, 1H),7.20-7.31 (m, 5H); HPLC (Short) t_(R) 2.29 min=0.07%, 3.88 min=2.68%,4.29 min=96.61%, 5.25 min=0.64%.

3. Preparation of BOC-Alkene:

A mixture of crude BMA (12.1 g, 35.2 mmol) prepared above and ethanol(240 mL) was heated to reflux and zinc dust (22.4 g, 343 mmol) wasadded. The resulting mixture was refluxed for 5 h, at which time TLCanalysis (silica gel, 30% EtOAc/Hexane) indicated the starting materialhad been consumed. The reaction mixture was cooled to room temperature,unreacted zinc dust was removed by filtration, and the filtrate wasconcentrated in vacuo to give an oil. This oil was dissolved in ethylacetate (100 mL) and washed with 2% aqueous acetic acid (50 mL). Theorganic layer was separated, dried (Na₂SO₄), filtered and evaporated togive 7.5 g of crude product, an oil; this oil solidified on standing atroom temperature to give a white solid. The solid was dissolved inmethylene chloride (50 mL) and the solution was filtered through 10 g ofsilica gel. Evaporation of the solvent gave 6.0 g (77% yield of thedesired olefin. HPLC analysis showed the olefin was >99% pure.BOC-Alkene: ¹H NMR (CDCl₃): δ 1.40 (s, 9H), 2.83 (br d, 2H, J=6.6Hz),4.43 (br s, 2H), 4.56 (br s, 2H), 5.06-5.13 (m, 2H, J=17.4, 10.5,1.2Hz), 5.8 (ddd, 2 H, J=17.1, 10.5, 5.4Hz), 7.20-7.31 (m, 5H); IR (thinfilm): ν3359 (NH), 1686 (CO), 1645 (alkene); HPLC (Short) t_(R) 3.87min=0.65%, 4.01 min=0.04%, 4.69 min=0.19%, 8.38 min=99.12%; MS, m/e MH⁺248.1661.

4. R,S-Epoxide by Alkene Route:

A mixture of BOC-alkene (0.498 g, 2.02 mmol), meta-chloroperbenzoic acid(1.93 g, 8.1 mmol) and dichloromethane (22 mL) was stirred at ambienttemperature for 3 h at which time HPLC analysis indicated the startingmaterial had been consumed. The reaction mixture was quenched withaqueous 10% Na₂SO₃ (60 mL), and diluted with diethyl ether. The organiclayer was washed with cold saturated Na₂CO₃ (60 mL), brine (60 mL),dried over Na₂SO₄, and the solvent evaporated to provide a clear oilthat solidified on standing. A white solid (0.49 g, 1.86 mmol) wasisolated in 92% yield and was shown to be a 5.2:1 mixture of R,S- andS,S-epoxide, respectively (HPLC, 96.5% pure combined). Analysis of theproduct mixture by proton NMR spectroscopy indicated an approximate5.7:1 ratio of diasteriomeric epoxides and no alkene starting material.

It is to be understood that the above description is intended to beillustrative and not restrictive. Many embodiments will be apparent tothose of skill in the art upon reading the above description. The scopeof the invention should, therefore, be determined not with reference tothe above description, but should instead be determined with referenceto the appended claims, along with the full scope of equivalents towhich such claims are entitled. The disclosures of all articles andreferences, including patent applications and publications, areincorporated herein by reference for all purposes.

1. A method for preparing an R,S-halomethyl alcohol (R,S-HMA) compoundhaving the following general formula:

said method comprising: reducing a halomethyl ketone (HMK) compoundhaving the following general formula:

with lithium aluminum t-butoxyhydride (LATBH) to form said R,S-HMAcompound; wherein: R¹ is an amino acid side chain; R² is a blockinggroup; and X¹ is a leaving group, wherein said leaving group is a halogroup; wherein: the reduction is carried out in diethyl ether.
 2. Themethod in accordance with claim 1, wherein R¹ is a member selected fromthe group consisting of a benzyl group, an S-phenyl group, an alkylgroup and para-nitrobenzene.
 3. The method in accordance with claim 1,wherein X¹ is chloro or bromo.
 4. The method in accordance with claim 1,wherein R² is a blocking group selected from the group consisting ofBOC, MOC and CBZ.
 5. The method in accordance with claim 1, wherein thereduction is carried out at a temperature ranging from about −30° C. toabout 25° C.
 6. The method in accordance with claim 1, wherein thereduction is carried out at a temperature ranging from about −5° C. toabout 5° C.
 7. A method for preparing an R,S-halomethyl alcohol(R,S-HMA) compound having the following general formula:

said method comprising: reducing a halomethyl ketone (HMK) compoundhaving the following general formula:

with a reducing agent selected from the group consisting of sodiumcyanoborohydride, cerium chloride/sodium borohydride, potassiumtri-sec-butylborohydride (K-Selectride®), potassium trisamylborohydride(KS-Selectride®) and (+)-B-chlorodiisopinocampheylborane ((+)-DipChloride™) to form said R,S-HMA compound; wherein: an amino acid sidechain; R² is a blocking group; and X¹ is a leaving group, wherein saidleaving group is a halo group.
 8. A method for isolating anR,S-halomethyl alcohol (R,S-HMA) from a mixture of R,S- and S,S-HMAs,said method comprising: combining the mixture of R,S- and S,S-HMAs withhexane and heating to a temperature ranging from 50° C. to about 60° C.to produce a hexane extractant, wherein said R,S-HMA has the followinggeneral formula:

wherein said S,S-HMA has the following general formula:

cooling said hexane extractant to a temperature ranging from about 0° C.to about 10° C., filtering said hexane extractant to form a firstretentate and recovering said first retentate; combining said firstretentate with hexane to form a hexane solution, heating said hexanesolution to a temperature ranging from about 50° C. to about 60° C., andcooling said hexane solution to a temperature ranging from about 30° C.to about 40° C. to produce a suspension; and filtering said suspensionto form a second retentate and recovering said second retentate, whereinsaid R,S-HMA is present in said second retentate.